Please see downstream bug https://bugs.gentoo.org/655552 for full details, but basically, sometimes when the steam icon is being drawn, it's being drawn over garbage left behind by the hexchat icon. I don't think it's steam's fault if xfce's notification area didn't clean up after hexchat. But no icons should be drawn (or redrawn after moving) on dirty backgrounds. In my opinion, a notification area icon should only be drawn on a clean, freshly cleared background.
Just to be clear, my distro told me to file it upstream here. If this issue has been fixed by now, I'll let them know to update their side.
*** This bug has been marked as a duplicate of bug 14577 ***
Thanks for taking the time to report this. We know about this problem already as it has been reported several times.